Using multi-vibrator circuit to pulse small electromagnetic coils

Discussion in 'The Projects Forum' started by NickB, Feb 1, 2016.

  1. NickB

    Thread Starter Member

    Feb 1, 2016
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    Hello there, this is my first post so please go easy on me.

    I want to create a circuit that will pulse two very small electromagnetic coils and have been experimenting with this multi-vibrator circuit on my breadboard using the same components but with 6v.

    http://www.technologystudent.com/elec1/dual1.htm

    My circuit is powered by 4 x AAA cells and is switched via a Mosfet connected to an RF receiver. The EM coils are very small air coils (6mm cores) with 50 winds of 28 gauge wire and appear to draw approx. 3.5 amps when connected direct to the batteries.

    Don’t laugh, but perhaps naively, I thought I could just swap out the 680R resistors and LEDs and replace them with the coils and snubber diodes. However, there doesn’t appear to be sufficient current to drive the coils at all.

    My question is - should I try to get more current to flow through the circuit by using higher spec transistors, or should I just use the multi-vibrator circuit to pulse two small relays connected to the coils and power instead?

    Space is an issue so I want to use as few components as possible.

    I’m a complete novice, but a quick learner. Any advice or help greatly appreciated.

    Thanks

    Nick
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    A 50-turn air-core inductor has very little inductance, so its AC impedance is very low. At DC it appears as a dead short to a 6V battery circuit. Without some other form of current limiting, the circuit will not function. What is this for?

    ak
     
  3. NickB

    Thread Starter Member

    Feb 1, 2016
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    Thanks for your reply.

    I build props for the magic community and the two coils in this prop will be used to make a diametrically opposed neodymium magnet rotate.

    The coils when connected direct to the batteries are quite powerful with more than enough pull.

    Thanks
    Nick
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here is the dilemma. The coil has a resistance of only about .4 ohms, so at 6 volts it would like to draw 15 amps, but the batteries can't supply that much so there voltage drops to around 1 or 2 volts.
    This might be okay if you don't need it to run very long, but the circuit won't work at the low voltage. We could add another battery to run a circuit say a 9 volt. Or we could add resistance in series with the coils to reduce the current, but both take some room. How tight are you for space? How long does it need to run between battery changes?
     
  5. NickB

    Thread Starter Member

    Feb 1, 2016
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    The entire project is to fit in a box approx 8 x 8 x 3 cm so batteries need to be flat - hence the 4 x AAAs. The circuit will be turned on via a tilt switch when in use for up to 30 mins at a time. The activation of the multi-vibrator will be via a keyfob remote (momentary type) as described earlier so it will be on for about one second max.

    The coils are used to pull a circular, diametrically opposed neodymium magnet towards it - alternating between the first and second coil is to eliminate dead spots if that makes sense. Hence why I thought a multi-vibrator or flip flop could work.

    Where you mention about reducing the current in the coils - I thought I'd need to increase the current in the coils to get them to work? Guess this is where my lack of knowledge is showing.

    Again, any help greatly appreciated to get this one across the finish line.

    Thanks
    Nick
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Maybe you could do an experiment.
    Try one of your coils on a single battery to see if it has enough power to pull your magnet.
     
  7. NickB

    Thread Starter Member

    Feb 1, 2016
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    Hi ronv, just tried it and a single AAA battery is just enough to pull the magnet. I've been using rechargeable cells which only put out about 1.3v each, however a new non-rechargeable battery puts out 1.6v which is fine.

    Thanks
    Nick
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Still thinking. :rolleyes:
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Maybe you could use 2 of these? Trouble is you have to buy a charger.
    http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=20386
     
  10. NickB

    Thread Starter Member

    Feb 1, 2016
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    Thanks for the reply.

    I'd looked at LiPo batteries and adding a charging jack isn't a problem if it comes to it.

    Not sure if it's a game changer, but I only need to pulse the first coil once followed by the second coil a second or so later so I guess it doesn't have to be a multivibrator or flip flop. However, the first coil must be off before the second coil is activated or it won't work. When I need to activate the 'gadget' again I'd just press the remote.

    Thanks
    Nick
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Hmmm, So just 1 revolution? Or is it a tick, tock?
    What does the signal from the remote receiver look like? Is it a pulse or does it stay on until you release the button?
     
  12. NickB

    Thread Starter Member

    Feb 1, 2016
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    The prop is essentially a dial with a pointer, a bit like a large compass - in a box. The diametrically opposed magnet is fastened to the centre of the pointer under the dial. When the coil is energised it causes the pointer to twitch and spin in a ghostly way - hence why I'm not using a motor, it also has to be silent. In experiments using the 4 x AAAs, just one pulse from the coil will cause the pointer to rotate fast several times. it's random where it stops but that is fine.

    The receiver is 5 - 10v (see link below) and when activated causes one of the pins to go high (this is connected to the gate on the Mosfet which I'm using as a switch). It stays on for however long I press the button on the remote keyfob.

    https://proto-pic.co.uk/simple-rf-m4-receiver-315mhz-momentary-type/

    Thanks
    Nick.
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I'm getting there.:D
    It's pretty easy to make it go one direction then immediately go the other direction.
    It is more difficult to make it go one direction, then pause then go the other direction.
    I'm not sure which one for sure you want.
    If it's the first, I think it would probably make a portion of a revolution and twitch at the end of it. Then reverse.
    If it is the second it might rotate and stop then reverse direction.
     
  14. NickB

    Thread Starter Member

    Feb 1, 2016
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    Hi Ronv, really appreciate your perseverance with this.

    I'd be happy to just get the pointer to rotate in any direction without dead spots. The effect is for it to look and behave in quite a random fashion. If you look at the attached diagram you will see the arrangement. If the circular magnet is in this position and coil 2 is energised it may twitch slightly but won't spin as it's in a dead spot position. If this coil goes off and coil 1 comes on it will pull North towards it causing the pointer above it to spin. That would be the ideal scenario. The effect would be used sparingly and may be repeated one more time (it's a spirit type effect where less is more).

    I have no control where the pointer/magnet will end up once it's finished turning, which is fine for the effect, but the use of two coils is important as I have to eliminate the dead spot in case it ends up in the position in the diagram.

    Also spotted this last night and wondered if the circuit Lee697 posted (the second one) could be adapted for use?

    http://forum.allaboutcircuits.com/t...r-in-the-same-circuit-using-a-trigger.101418/

    Thanks
    Nick
     
  15. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here is a cheap and dirty try. See if it is what you want.
    The output of your receiver goes minus and turns on the PFET switch you already have.
    This turns on the first coil. After it turns off the second one turns on.
     
  16. NickB

    Thread Starter Member

    Feb 1, 2016
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    Hi Ronv, many thanks for all your help with this, I do appreciate it, but this is way too complex for me. Gulp!

    I'm struggling to even understand what some of the components are (inverter chips?), let alone what they do. Looks like you simulated it so I'm sure it works but I wouldn't have a clue how to build this - and I have ten to make!

    Think I need something much simpler - if that feasible?
     
  17. NickB

    Thread Starter Member

    Feb 1, 2016
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    The more I look at the circuit and Google the components the less scary it's becoming. The components appear inexpensive - just a lot of them! I presume the inverter is just one chip?

    Because I intend to make a few of these I was hoping for something that wouldn't involve a PCB board, something I could just solder together in a wire loom and glue in place with a hot glue gun.

    Thanks
    Nick
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If it were me I'd try repositioning the coils so that they're 90* apart rather than180* apart and wire them in series. I think that would eliminate any dead spot and so enable just a single pulse to twitch/spin your gizmo.
     
  19. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, not as bad as it looks & yes there are 6 inverters in 1 chip.
    They make "breadboards that make it pretty easy to wire with 30 awg wire.
    [​IMG]

    @Alec_t 's idea is worth a try, don't you think?
     
  20. NickB

    Thread Starter Member

    Feb 1, 2016
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    Thanks Alec_t, I'd considered it when I started the project but assumed any difference in the coils' strength may be an issue so didn't try it. I'm going to give it a go this weekend as it would save a lot of time and components. Thanks for bringing it to my attention though.

    Nick
     
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